Ken:  I have had a VDO Vacuum gauge attached to my 79 loaf and my
Vanagon motors and can speak to what normal is for a Vanagon.  The
Vanagon of mine normally idles when warm at 15" of Hg and non full
shut down of the throttle going down a steep hill will generate about
22-23".  Can't quite get the gauge to get to zero under full
throttle, but that could be an age thig with the gauge.  I've had it
on like my last 3 cars.  Not only is a vacuum gauge to me more
important than looking at voltage or amperage, but it also helps me
minimize pressure on the throttle for climbing hills and saving some
fuel maybe.

My 79 loaf engine had dual two barrel Webers on it with individual
intakes obviously and I tried running the brake booster off one
runner and the vacuum gauge off the other and I still got stable idle
readings, al;though not as stable as  the Vanagon.  I attributed that
to the 3/4 Ray Litz camshaft in there which will not make for the
smoothest idle or vac. readings.  tying the  3/4 side runners
together made no difference in gauge reading or idle, so I went back
to segregating the ports.

  According to the Haynes manual for our Toyota, a slow wide range of
drop like the 0-20 you speak of is either clogged PVC(which you
likely don't have), or incorrect idle fuel mixture, or a throttle
body or intake gasket leak.

YMMV

DM&FS

At 11:20 PM 8/21/2009, Ken Lewis wrote:
>    I have searched the internet but can not find the answer.  All web sites
>state a "normal" engine's vacuum, at idle, is ~ 17 to 21 inches at the
>intake manifold. But, all these examples are for cars with one manifold
>connected to at least four cylinders. I am trying to diagnose a flat four
>with four venturis and individual intake manifolds. When I  attach the
>vacuum gauge under a single venturi it produces rapid swings from zero to
>20. Do I need to connect all test ports together to smooth these readings
>out or is this the problem?
>TIA
>Ken